The affinity between avidin and biotin or between streptavidin and biotin is very high (Kd=10−15 to 10−14 M) and provides one of the strongest in vivo binary interactions. At present, avidin/streptavidin-biotin interaction is widely used in the field of biochemistry, molecular biology or medicine (Green, (1975), Adv. Protein Chem., 29: 85-133; Green, (1990), Methods Enzymol., 184: 51-67). Avidin is an albumen-derived basic glycoprotein and has an isoelectric point exceeding 10. Due to its high basicity or sugar chain, avidin involves a problem of non-specific binding to DNA or other molecules, which limits the use of avidin. On the other hand, streptavidin is derived from an actinomycete strain (Streptomyces avidinii), and it has an isoelectric point at around neutral pH and contains no sugar chain. These two proteins each form a tetramer and bind to one biotin molecule per subunit. Their molecular weight is around 60 kDa.
Tamavidin (tamavidin 1; SEQ ID NO: 2) is a third biotin-binding protein that was purified from an edible mushroom, Pleurotus cornucopiae var. citrinopileatus, as an antifungal protein against the rice blast disease pathogen M. grisea. This protein has also been determined for its gene structure (WO 02/072817). Its homolog (tamavidin 2; SEQ ID NO: 4) has also been identified from the same mushroom, and recombinant proteins have also been successfully produced (WO 02/072817). Tamavidin homolog can be readily produced by being expressed in E. coli cells and purified with an iminobiotin column. This is a great advantage over the production systems for avidin and streptavidin.
When avidin is expressed in E. coli cells, the yield of a soluble protein is as low as about 50 μg per 50 ml (Airenne et al., 1994, Gene, 144: 75-80). For this reason, insect cell systems using baculoviruses are used currently (Airenne et al., 1997, Protein exp. Purif., 9: 100-108). Likewise, when streptavidin is expressed in E. coli cells, the resulting recombinant protein will form an insoluble inclusion body. This inclusion body should be solubilized with a high concentration of guanidine hydrochloride, followed by stepwise removal of guanidine hydrochloride through dialysis to cause protein refolding, thereby obtaining a soluble and active recombinant streptavidin (Sano and Cantor, 1990, Proc. Natl. Acad. Sci. USA, 87: 142-146). In this way, the production of avidin and streptavidin requires a great amount of effort and time. In contrast, when a tamavidin homolog was expressed in E. coli cells, 1 mg recombinant protein was obtained per 50 ml culture. This corresponds to a high level of efficiency in the production of biotin-binding proteins, and indicates the potential usefulness of a tamavidin homolog protein.
In the fields of reagents and diagnostic agents, there have been developed solid supports (e.g., magnetic beads, microplates or sensor chips) having avidin or streptavidin attached thereto. However, there is no report of such solid supports with low levels of non-specific binding and with high stability at a higher temperature range.    Patent Document 1: International Publication No. WO 02/072817    Non-patent Document 1: Green, 1975, Adv. Protein Chem., 29: 85-133    Non-patent Document 2: Green, 1990, Methods Enzymol., 184: 51-67    Non-patent Document 3: Airenne et al., 1994, Gene, 144: 75-80    Non-patent Document 4: Airenne et al., 1997, Protein exp. Purif., 9: 100-108    Non-patent Document 5: Sano and Cantor, 1990, Proc. Natl. Acad. Sci. USA, 87: 142-146